This invention relates to intrusion detection systems in general and more particularly to an automatic sensitivity adjustment for such a system.
A major problem in intrusion detection systems is due to incorrect adjustment or alignment of the equipment as will be explained. In many such systems, if the equipment is improperly adjusted or aligned, a false alarm will occur. This creates great inconvenience both to the user and the manufacturer.
The adjustment and alignment of any such system requires proper and careful installation. Hence, prior art relied upon the installer to determine that an adequate margin of sensitivity was maintained in the system during installation. If the systems do not operate with a given safety factor, a borderline condition could occur where the system would be placed either in alarm or normal condition due to changes in ambient. The ambient conditions can vary widely and can serve to effect the background noise and hence, must be accounted for before the detection system will denote a true alarm. For example, such ambient noise conditions may occur due to expansion and compression of structural members contained in the building and so on.
Typical of this type of problem is an intrusion system which employs a photoelectric beam. The photo electric beam must be aligned in such a manner that there is an adequate amount of light emanating from the transmitter and focused at the receiver. The amount of light must be in excess of the level for marginal operation. Otherwise, a false alarm will occur when the photo electric receiver sensitivity is changed by ambient conditions which encompass natural phenomenon. Such changes as indicated above, may also include ambient light level changes, temperature changes, component aging, structural changes and so on.
While the problem is prevalent in a photo electric beam intrusion system, similar problems occur in ultrasonic systems, microwave systems and so on. Accordingly, the problem can be characterized in that there must be an adequate margin between an alarm and a normal condition, which margin must consider and take into account the above noted ambient conditions.
While the prior art was cognizant of these problems, the margin of safety was determined primarily by the installer. In this manner, the installer was responsible in determining the ambient problems so that he could adjust the sensitivity of the system to assure that the system would operate to provide adequate protection. If the installer was wrong, one would either experience a high percentage of false alarms or the sensitivity of the system would be reduced to a point which would result in marginal protection of the area.
The problem is recognized in U.S. Pat. No. 3,838,408 entitled ENVIRONMENTAL TEST SWITCH FOR INTRUDER DETECTION SYSTEMS issued on Sept. 24, 1974 to L. McMaster and assigned to Detection Systems, Inc. In this system, a switch was provided for the installer to access. This switch was designated as an environmental test switch and when activated, served to increase the systems sensitivity by a desired factor. When this was done, the installer adjusted the gain of the receiver to a level just below that which produced an alarm. When the switch was turned off or inactivated, the sensitivity was set at a proper level for a given margin of safety. In any event, this system again required the installer to depress the switch and to make the necessary adjustments. Accordingly, changes in environmental conditions which would lead to false alarms would not necessarily be identified in the above noted system.
It is therefore an object of this invention to provide an automatic sensitivity adjustment for a receiver which therefore eliminates the necessity of installation adjustments.